One or more users are trying to send emails to a specific domain via Symantec's mail servers but these emails are going into retry as the Email Security.cloud service is unable to connect to the recipient's infrastructure.
421 4.4.0 [internal] no MXs for this domain could be reached at this time
Note that the "MXs" part in error message "421 4.4.0 [internal] no MXs for this domain could be reached at this time" does not refer to MX records in context but rather stands for "Mail Exchanger".
This is could be caused by an Intrusion Prevention System inspecting SMTP traffic, using specific SMTP rules such as:
Block IP address that exceeds unknown user delivery count
Block IP address that gets denied for relaying too often
Block IP address that exceeds RSET session count
Block IP address that exceeds message spam score
Block IP address that gets listed on DNSBL
Block IP address that exceeds message size
An intrusion prevention system can also have different actions on the amount of time an IP address can be blocked for, such as: